Semiconductor laser diodes were originally fabricated in a manner that led to a diode structure that was parallel to the surface of the semiconductor wafer. In this structure, light is emitted from the edge of the structure such that the light is also emitted parallel to the surface of the semiconductor wafer. Unfortunately, this structure does not lend itself to low cost "mass" manufacturing or to the cost-effective fabrication of two-dimensional arrays of laser diodes.
A second class of laser diodes is fabricated such that the laser structure is perpendicular to the surface of the semiconductor wafer and the light is emitted perpendicular to the surface. These laser diodes are commonly known as Vertical Cavity Surface-Emitting Lasers (VCSELs). A VCSEL may be viewed as a laser having mirrors constructed from alternating layers of material having different indices of refraction. These lasers are better suited for the fabrication of arrays of lasers for displays, light sources, optical scanners, and optical fiber data links. Such lasers are currently being considered for use in CD-ROM drives, DVD heads, and laser printers.
To provide small diameter light beams while maintaining efficient conversion of electrical energy to light, the current flowing vertically in the VCSEL must be confined to a small area. Early designs utilized ion-implanted regions to contain the current flow; however, this approach is not satisfactory for very small confinement areas. In addition, implanted regions do not offer any index-guided optical confinement.
A method for containing the current flow and also for providing optical confinement is to convert a portion of one or more of the mirror layers in the upper mirror to an electrical insulator. These VCSELs utilize an oxidation process to convert one or more high aluminum content layers within the VCSEL structure to some form of aluminum oxide. The oxidation process proceeds along the layer from the outer edge of an etched mesa toward the center of the mesa. The process is stopped prior to converting the entire layer, thereby leaving a small unoxidized area in the center of the mesa, which defines the laser aperture.
One problem with these native-oxide VCSELs is the nonplanar geometry currently employed in fabricating such devices. To provide access to the layer being oxidized, the device is first etched to form a mesa-like structure with the edges of the various mirror layers exposed. The exposed edges are then subjected to the wet oxidation process.
If a number of lasers are to be constructed on a common substrate, this process leads to a device in which each laser is an isolated mesa on the substrate. The surface of the device has large steps which make the deposition of the various metallic conductors, such as those used for contacts, difficult. In addition, the need to provide trenches between the devices limits the density of lasers that can be fabricated in array structures. Even in the case of single devices, the raised mesa structures make it difficult to deposit metal for the connections to the electrode on top of the mesa.
Broadly, it is the object of the present invention to provide an improved VCSEL structure and method for fabricating the same.
It is a further object of the present invention to provide a VCSEL structure that provides a near planar top surface.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.